git checkout -b readme-update is a shortcut command that allows you to combine the creation of the branch (git branch readme-update) and checking out to that branch (git checkout readme-update). The -b tells Git to create a new branch.

git push -u origin readme-update is the slightly longer version of the push command that should be used when you push a new branch for the first time.

The -u is the short version of the option --set-upstream. This option tells Git to create a relationship between our local branch and a remote tracking branch of the same name.\

You only need to use this long command the first time you push a new branch. After that, you can simply use git push.